An article published in the journal “Proceedings of the National Academy of Sciences” describes the possible role of transposons, commonly called jumping genes, in the emergence of eukaryotic organisms, the ones formed by complex cells that also include multicellular ones. A team of researchers conducted experiments in particular on retrotransposons, which to integrate into DNA require an RNA intermediate, to assess their effects on bacteria. The result is that a DNA repair mechanism typical of eukaryotes but rare in bacteria increases the efficiency of insertion and the effect of retrotransposons suggesting a possible role in the evolution of eukaryotes.
The idea for this research starts with a thought by Nigel Goldenfeld of the University of Illinois and director of NASA’s Institute for Universal Biology and Thomas Kuhlman, now at the University of California, Riverside. The two researchers wondered why in particular retrotransposons are so common among multicellular organisms while they’re rare in bacteria.
The study of jumping genes began in the 1940s thanks to Barbara McClintock’s discoveries but only in recent years, thanks to advances in the field of genetic techniques, made real leaps forward. That’s why only now the two researchers, together with other collaborators, were able to try to insert their genes into the DNA of bacteria to see what would happen.
The result can be lethal but depends on some factors. When retrotransposons are inserted into the DNA of a bacterium, in very simple words they perform a cutting and sewing action. Bacteria such as the ones belonging to the E. coli family have only one way to sew DNA, repairing it. Eukaryotic organisms and some bacteria have another way called nonhomologous end-joining (NHEJ). In bacteria, DNA repair isn’t enough to save them from the changes made but in eukaryotes there are more complex interactions between the repair mechanism and retrotransposons.
The crucial difference is that complex organisms have the spliceosome, a complex formed by RNA molecules that manage the so-called junk DNA. Some parts are similar to a group of genetic sequences called introns, existing in bacteria. The researchers hypothesize that there was an evolution of the genetic mechanisms of regulation of transposons and retrotransposons that allowed the NHEJ mechanism to add retrotransposons without killing the organism and contributing to its evolution.
It’s possible that these mechanisms were a key to the evolution of eukaryotes when they became sophisticated enough to withstand changes to DNA. Having a greater amount of retrotransposons led to more changes in DNA and the ones that benefited the organism got widespread due to natural selection.
Nigel Goldenfeld offered some reflections on the transition between microbial life and complex life, also in his role as director of NASA’s Institute for Universal Biology and therefore thinking about the possibilities of extraterrestrial life. Lab experiments can help to understand the possibilities of evolution of complex organisms on Earth and perhaps on other planets. It’s another reason why this hypothesis is really intriguing and will keep on being explored by the researchers.